Roll-up doors are used in industrial or commercial establishments to separate different areas of a building, or to separate the inside of a building from the exterior. For example, roll-up doors may be used to separate two areas or zones which have different temperature or humidity conditions or to provide noise control between two areas. A roll-up door has the advantage of being capable of being moved rapidly between the open and closed positions, and when in the open position, the door panel is wound on a drum located above the doorway, so that the door panel will not obstruct the doorway.
U.S. Pat. No. 3,878,879 describes a roll-up door incorporating a counterbalancing system. In that patent, a drum is mounted above the doorway and a flexible door panel is wound on the drum. A pulley is secured to each end of the drum and one end of a cable is secured to the pulley, while the opposite end of the cable is connected to the bottom of the door panel. Secured to each cable intermediate its ends is a weight which serves to counterbalance the weight of the door panel. If additional tensioning is required for the door panel, when in the closed position to prevent wind deflection, it is necessary to add additional counterweight. However, added counterweight cannot be easily implemented and requires increased driving force to move the door panel to the closed position.
U.S. Pat. No. 5,048,588 is directed to a roll-up door incorporating a counterbalancing mechanism and a separate spring tensioning mechanism. In accordance with that patent, a pair of pulleys are secured to each end of the drum and one end of a first belt is secured to a first of each pair of pulleys, while the opposite end of the belt is dead-ended on the frame of the door. A counterbalancing mechanism, such as a counterweight, is attached to the belt intermediate its ends and exerts a rotational force on the drum in a direction to wind the door panel on the drum.
In addition, one end of a second belt is secured to a second of each pair of pulleys and the opposite end of each second belt is connected to the lower end of the door panel. A resilient member, such as a spring, is connected to each second belt intermediate its ends and exerts a force on the door panel to stretch the door panel at all positions. The force of the spring can be adjusted to regulate the tensioning of the door panel. With the construction of the aforementioned patent, the door panel tensioning system is separate from the counterbalancing system, so that the tension on the door panel can be controlled without affecting the counterbalancing.